For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Multi-strand tensioning is used when forming especially long post-tensioned concrete structures, or those which must carry especially heavy loads, such as elongated concrete beams for buildings, bridges, highway overpasses, etc. Multiple axially aligned strands of cable are used in order to achieve the required compressive forces for offsetting the anticipated loads. Special multi-strand anchors are utilized, with ports for the desired number of tensioning cables. Individual cables are then strung between the anchors, tensioned and locked as described above for the conventional monofilament post-tensioning system.
As with monofilament installations, it is highly desirable to protect the tensioned steel cables from corrosive elements, such as de-icing chemicals, sea water, brackish water, and even rain water which could enter through cracks or pores in the concrete and eventually cause corrosion and loss of tension of the cables. In multi-strand applications, the cables typically are protected against exposure to corrosive elements by surrounding them with a metal duct or, more recently, with a flexible duct made of an impermeable material, such as plastic. The protective duct extends between the anchors and in surrounding relationship to the bundle of tensioning cables. Flexible duct, which typically is provided in 20 to 40 foot sections is sealed at each end to an anchor and between adjacent sections of duct to provide a water-tight channel. Grout then may be pumped into the interior of the duct in surrounding relationship to the cables to provide further protection.
Several approaches have been tried to solve the problem of quickly, inexpensively and securely sealing the joints between adjacent sections of duct used in multi-strand post-tensioned applications. However, all prior art devices have utilized a plurality of arcuate sections which must be assembled at a joint around the ends of adjacent duct sections. Wedges, compression bolts or the like then are used to compress the joined sections into sealing engagement with the duct and with each other. Such prior art devices have been cumbersome to use and have proved somewhat unreliable in their ability to exclude moisture or other corrosive elements from the interior of the ducts.
Several patents have issued relating to duct couplers. For example, U.S. Pat. No. 5,320,319, issued on Jun. 14, 1994, to K. Luthi describes a coupling element which is fitted with chamfered flanges. The sheaths of the coupler have protrusions which are inserted into the coupling element with a tubular element which forms the end of the sheaths. A sealing ring is inserted between each of the flanges and protrusions of the sheaths. The flanges and the protrusions are held together by sloping surfaces and by a groove worked within each socket. Also, U.S. Pat. No. 5,474,335, issued on Dec. 12, 1995, to the present inventor, describes a duct coupler for joining and sealing between adjacent sections of the duct. The coupler includes a body, flexible cantilevered sections on the end of the body adapted to pass over annular protrusions on the duct and locking rings for locking the cantilevered flexible sections into position, so as to lock the coupler onto the duct.
U.S. Pat. No. 5,775,849, issued on Jul. 7, 1998 to the present inventor, describes a coupler as used for ducts in post-tension anchorage systems. This duct system includes a first duct having a plurality of corrugations extending radially outwardly therefrom, a second duct having a plurality of corrugations extending radially outwardly therefrom, and a tubular body threadedly receiving the first duct at one end and threadedly receiving the second duct at the opposite end. The tubular body has a first threaded section formed on an inner wall of the tubular body adjacent one end of the tubular body and a second threaded section formed on the inner wall of the tubular body adjacent an opposite end of the tubular body. The threaded sections are formed of a harder polymeric material than the polymeric material of the first and second ducts. The tubular body has an outer diameter which is less than the diameter of the ducts at the corrugations. The first and second threaded sections have a maximum inner diameter which is less than the outer diameter of the ducts at the end of the ducts. First and second elastomeric seals are affixed to opposite end of the tubular body and juxtaposed against a surface of a corrugation of the first and second ducts.
U.S. Pat. No. 5,954,373, issued on Sep. 21, 1999 to the present inventor, describes a different type of duct coupler apparatus. The duct coupler apparatus of this patent includes a tubular body with an interior passageway between a first open end and a second open end. A shoulder is formed within the tubular body between the open ends. A seal is connected to the shoulder so as to form a liquid-tight seal with a duct received within one of the open ends. A compression device is hingedly connected to the tubular body for urging the duct into compressive contact with the seal. The compression device has a portion extending exterior of the tubular body. The compression device includes an arm with an end hingedly connected to the tubular body and having an abutment surface adjacent the end. The arm is movable between a first position extending outwardly of an exterior of the tubular body and a second position aligned with an exterior surface of the tubular body. A latching member is connected to an opposite end of the arm and serves to affix the arm in the second position. The abutment surface of the arm serves to push a corrugation of the duct against the seal and against the shoulder so as to form a liquid-tight seal between the duct and the interior of the coupler.
It is an object of the present invention to provide a coupler for sealing between adjacent sections of an elongated duct.
It is another object of the present invention to provide a coupler which facilitates installation by the user and which, when engaged with the opposed ends, will securely seal against the intrusion of corrosive elements and to prevent the leakage of sealing materials from the interior of the duct.
It is a further object of the present invention to provide a coupler which includes compressible seals for securely engaging coupler sections together and for conforming the seals to the surfaces of the duct.
It is another object of the present invention to provide a coupler which is easy to use, easy to manufacture and relatively inexpensive.
It is still a further object of the present invention to provide a duct coupler apparatus which maintains the integrity of an annular seal in the area of the connections between the coupler and the duct.
It is still another object of the present invention to provide a duct coupler apparatus which prevents the ducts from longitudinally separating from each other.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.